Wanted to do a playthrough as a thalmor agent. I'm just curious what triggers thalmor aggression.

Obviously don't attack or question the random thalmor patrols.

And I know elenwen sends a patrol after you after the thalmor embassy mission.

Is there any other quests or events that send the thalmor after you?

The "aggressive" Thalmor agents are one of the wilderness encounter quests in the game (WE34 in the Creation Kit). The chance of them happening is random and is not directly tied to the main quest. This encounter is set to only occur in Imperial controlled (or neutrally aligned) holds.

It is rumored to be as a result of the player being "Dragonborn", whose existence alone is supposedly a threat the Thalmor's interests.